In a typical wireless, cellular or radio communications network, wireless devices, also known as mobile stations, terminals, and/or User Equipment, UEs, communicate via a Radio-Access Network, RAN, with one or more core networks. The RAN covers a geographical area which is divided into cells, with each cell being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB”, “eNodeB” or “eNB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. One radio base station may serve one or more cells.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio-access network, UTRAN, is essentially a RAN using wideband code-division multiple access, WCDMA, and/or High-Speed Packet Access, HSPA, to communicate with user equipment. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN, as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio-Access Network, E-UTRAN, also known as the Long-Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio-access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base station nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio-Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
FIG. 1 shows an example of a network access procedure performed in a wireless communications network for a wireless device. In this example, the wireless communications network is based on EPS/LTE. However, other wireless communications network based on other wireless communication technology may also comprise a similar network access procedure.
The network access procedure starts with a random access, RA, procedure for synchronizing with and gaining initial access to the wireless communications network, as shown in FIG. 1 by Actions 101-104. The RA procedure also serves the purpose of assigning the wireless device with a unique identity when establishing an initial radio link to a network node serving a cell in the wireless communications network. The network access procedure also comprises a Radio Resource Control, RRC, connection establishment procedure which serves to perform authentication, to configure the connection, and to establish appropriate states on higher layers. The RRC connection establishment procedure may be said to actually start in Action 103 in FIG. 1, but is continued in Actions 105-1028. After the RRC connection establishment procedure, the wireless device has transitioned from a RRC_IDLE state to a RRC_CONNECTED state in the wireless communication network and may begin transmitting and receiving data, as shown in FIG. 1 by Actions 1029-1034. Note that Actions 105-1034 are not further described herein, but may e.g. be found in the standard 3GPP TS 23.401 V13.1.0 (2014-12), section 5.3.2.
As shown in the example of FIG. 1, the RA procedure in the wireless communications network may comprise the following actions 101-104.
Action 101
The wireless device, denoted UE in FIG. 1, transmits a RA preamble on the Physical Random Access Channel, PRACH to the network node, denoted eNB in FIG. 1. This message is commonly denoted RA Msg1. Each cell in the wireless communications network may have its own set of RA preambles. However, RA preambles may also be reused between cells, but preferably not in adjacent cells. Optionally, the RA preambles may also be divided into two groups, e.g. group A and group B. In this case, the UE may then select the group from which to pick a preamble, e.g. at random, based on the potential message size, i.e. the potential message size being the data available for transmission in Action 103 plus MAC header and any possible MAC control elements, and the channel quality, e.g. estimated in terms of the measured downlink path loss. Here, two conditions may be met for the wireless device to select a preamble from group B, i.e. the potential message size has to exceed a certain threshold and the estimated path loss has to be lower than a certain threshold.
Action 102
In response to the RA preamble, the network node transmits a Random Access Response, RAR, to the UE using a broadcast identifier, such as, for example, a Random Access Radio Network Temporary Identifier, RA-RNTI. The RAR also includes an uplink, UL, grant, i.e. an allocation of uplink transmission resources, for the wireless device. This message is commonly denoted RA Msg2.
The RAR Packed Data Unit, PDU, may comprise a back-off indicator and zero or more Medium Access Control, MAC, RAR. Each MAC RAR contains a temporary identifier, i.e. a Temporary Cell Radio Network Temporary Identifier, TC-RNTI. Each MAC RAR further contains a timing advance command, an uplink grant and a reserved bit. The MAC PDU header contains one MAC sub-header, i.e. Random Access Preamble ID, RAPID, sub-header, for each MAC RAR that is included in the RAR PDU. Each such corresponding MAC sub-header, or RAPID sub-header, includes a RA preamble identifier which indicates the received RA preamble that the corresponding MAC RAR pertains to. Hence, in this way each MAC RAR is mapped to a RA preamble that is transmitted by the wireless device and received by the network node in Action 101 and PRACH resource.
Action 103
Here, the wireless device transmits an RA message containing a pre-set identity in a RRCConnectionRequest message. This message is commonly denoted RA Msg3. In a Frequency Division Duplex, FDD, mode, this RA Msg3 may be transmitted 6 or 7 subframes after the reception of the RAR in Action 102 depending on the parameters in the UL grant received in Action 102. In a Time Division Duplex, TDD, mode, the timing also depends on the configuration of UL and downlink, DL, subframes.
The pre-set identity which the wireless device includes in the RRCConnectionRequest message is a SAE Temporary Mobile Subscriber Identity, S-TMSI, if the S-TMSI is available. The S-TMSI is a 40-bit determined identity, which is assigned by the Mobility Management Entity, MME, and which consists of the MME Group ID, MMEGI, and the MME Code, MMEC. This is typically the case unless the wireless device is accessing the wireless communications network from a DETACHED state, e.g. when the wireless device is turned on. Alternatively, the pre-set identity which the wireless device includes in the RRCConnectionRequest message may, if no S-TMSI is available, be a 40-bit random number.
Action 104
In response to the RA message in Action 103, the network node transmits the identity of the wireless device to be used in the cell on the DL together with an RRCConnectionSetup message. The identity of the wireless device may be contained in a UE Contention Resolution Identity MAC Control Element or as a parameter in the RRCConnectionSetup message. This message is commonly denoted RA Msg4.
In case of possible RA preamble collisions of two or more wireless devices, the result of the contention resolution in the network node is communicated through the above mentioned identity of the wireless device in this RA Msg4. It should also be noted that the RRCConnectionSetupComplete message in Action 105 is herein considered to not form a part of the RA procedure, but may still be a part of the RRC connection establishment procedure when the RA procedure has been concluded.
Contention resolution in a network node serves to resolve a situation where two or more wireless devices in the same cell happen to use the same RA preamble in the same PRACH resource, e.g. in Action 101. In this case, the two or more wireless devices will both assume that they are the intended recipient of the RA message in Action 102 from the network node, i.e. RA Msg2. Consequently, both of the two or more wireless devices will send an RRCConnectionRequest message in RA Msg3 to the network node in Action 103. The network node will then at best correctly receive one of these messages. The network node may then indicate which of the two or more wireless devices that it is responding to by including the identity of the wireless device, for which it correctly received the RA Msg3, in the RA Msg4.
However, in a worst case scenario, the network node may not be able to receive any one of the RA Msg3 messages. In this case, all of the two or more wireless devices using the same RA preamble in the same RA resource, i.e. PRACH resources, will fail to access the wireless communications network. A wireless device that fails the RA procedure has to restart the RA procedure. This has several adverse consequences, for example, increased radio resource consumption which puts a further strain on the RA resources, increased RA load, increased interference, increased processing load in the network node, delayed network access and increased energy consumption in the wireless device.
One option to solve this issue would be to largely increase the resources available in the wireless communications network for the RA procedure. However, this is not a very practical or economically feasible option, since this would wastefully over-dimension resources which may only be fully used during occasions of high network access loads.